With the advent of independent voltage islands on integrated circuits (especially ASICs) and the potential to turn on and off these voltage islands as performance, power, and heat require, the problem exists of how to turn on and off the power without disrupting the voltage supply to other sections of the integrated circuit.
When a voltage island is turned on or off, a part of the integrated circuit has either the power turned on or turned off. This means that the power grid of the circuit has a capacitive load either connected or disconnected from the grid. This causes the power grid voltage level to bounce around. Usually this variation can be represented by a decaying sinusoidal waveform.
In FIG. 1, the curve 105 shows the voltage level close to the point that the power is switched on or off. In this example, line 100 represents the nominal voltage and line 110 represents a high voltage limit that will significantly impact other portions of the circuit, either by providing incorrect results, changing timing, etc. Arrow 120 indicates a rapidly varying period when a switch may put the voltage out of range. Arrow 130 represents a sub-range when the voltage is falling rapidly. A switch at this time that decreases the voltage may put it out of range, but a switch that increases the voltage will tend to reduce the effect of the rapid decrease.
Those skilled in the art will appreciate that a switch in the period indicated by arrow 120 may either increase or decrease out of range excursions, but that the voltage is changing on a very fast time scale and it is risky to switch another island so soon after the switch that produced curve 105.
As multiple voltage islands are switched on and off, performance requirements change within the integrated circuit. These sinusoidal decaying voltage levels from the various voltage islands are added together. This results in a voltage level that varies over time and is dependent on performance requirements and capacitive loads such as voltage islands being switched on and off the grid. The curve in FIG. 1 has been selected for ease in explanation to show the response of the grid to a single switching incident.
In the past this voltage level (either too high or too low) was handled by evaluating the worst case performance requirements and then over specifying a voltage grid structure that would guarantee that the voltage would stay within limits.
This problem has changed with the advent of voltage islands. The use of power switchable voltage islands means that the capacitive load of a major portion (⅓, ¼, ½, ⅕, etc.) of the integrated circuit could be switched at a given time. Trying to over specify/design the power grid to handle these fluctuations would be too costly in terms of area used for bypass capacitances and wiring layout.
Assuming that a more sophisticated approach of selecting the time and/or quantity of load to be switched is available, the problem arises of choosing parameters that represent a reasonable tradeoff between resource use and performance.
The art could benefit from a method of selecting parameters to permit or forbid a switch that provides an acceptable degree of circuit protection while consuming a tolerable amount of system resources.